The present invention generally relates to composite materials and tensile coupon testing and, more particularly, to carbon-reinforced composite materials and a test method for visual documentation of the formation and growth of micro-cracks in these materials during tensile coupon testing.
The requirements for material used in the aerospace industry are manifold. Demands include improved toughness, lower weight, as well as increased resistance to fatigue and corrosion. The boundaries of material properties are being constantly extended as manufacturers strive to give the next generation of aircraft improved performance while making them more efficient. The use of fiber-reinforced composite materials over metals and their alloys over the past 20 years has increased significantly because of the weight savings and the improvement in fatigue life and corrosion control. Fiber-reinforced composite materials contain a strong and stiff fiber, such as a carbon fiber, embedded in a softer matrix material, such as a resin. Laminated composite materials generally exhibit an initial stiffness that is used in the design of structures. Laminated composites are constructed of many layers of fiber-reinforced materials. Fiber-reinforced polymer matrix composites, such as carbon (graphite)/epoxy and carbon/cyanate ester are now the materials of choice for spacecraft and launch vehicle structures and subsystems such as optical benches, instruments, and antennas. Furthermore, fiber-reinforced polymer matrix composites are widely used in commercial and military aircraft, sports equipment, and industrial and medical equipment. For example, composite structure utilization on a AV-8B Harrier strike aircraft is about 28% by weight, and composite structure utilization on a F/A-18 E/F Super Hornet strike fighter is about 20% by weight. Application of composite materials, such as carbon reinforced composite materials, is particularly valuable, because of performance improvements, component mass reduction, and relatively low manufacturing costs.
Many fiber-reinforced composite materials experience time-dependent micro-structural damage at loads far below their failure stress. Damage here refers to ply-level micro-structural changes such as matrix cracking and fiber—matrix debonding. Such micro-cracking can have significant softening effect in the off-axis fiber directions and may cause more serious forms of damage, such as transverse cracking, delamination, and finally fiber failure. As the load on fiber-reinforced composite materials increases, the effective stiffness may be reduced and an onset-of-damage may be observed which is caused by internal micro-cracks in the resin. The fatigue life capability of a composite material and, therefore, the capability to withstand repeated loading, are expected to be limited by the onset-of-damage, the micro-cracking. On the other hand, almost infinite fatigue life for repeated loading of carbon-reinforced composites is possible if the loading intensity is held below the initial formation of micro-cracks (onset-of-damage), as shown in FIG. 1. FIG. 1 (prior art) provides an x-y plot 10 showing the fatigue life of a carbon/epoxy composite material (trace 11) in comparison with metals and alloys (traces 12, 13, and 14). Consequently, it is very important to have a reliable method that can experimentally determine the presence and extent of micro-cracking.
To date, the best method to find micro-cracks is to apply a certain load of increasing load levels to many specimens (tensile coupon testing), to destructively cross section each of these specimens, prepare photomicrographs, and visually check for micro-cracks. Such method has been described, for example, by Scott T. Burr, David L. Sikarskie in “Damage Characterization of Woven Composites Under Static Tensile Stress”, 1995 SEM Spring Conference and Exhibit, June 12–14, Grand Rapids, Mich., U.S.A. Despite the enormous time and test material expense, this method is not always successful since the micro-cracks may close when the load is released. Furthermore, the cross section may not have been taken at the exact location where micro-cracks exist.
Other prior art approaches to determine micro-cracking in composite materials include the acoustic emission method. During the application of a load to a composite material specimen, acoustic events emitted from a micro-crack can be recorded. Since the acoustic wavespeed changes with the fiber direction, the relative arrival time of an acoustic event emitted from a micro-crack may not be identical with the formation of the micro-crack. While the dynamic growth of a micro-crack can be monitored using the acoustic emission method, this method cannot reliably determine the initial formation of micro-cracks.
As can be seen, there is a need for a method to reliably determine the presence and extent of micro-cracking in carbon-reinforced composite materials. Furthermore, there is a need to experimentally determine the load that causes the initial formation of micro-cracks (onset-of-damage). Also, there is a need to obtain test results without destructive testing of many specimens.
There has, therefore, arisen a need to provide a method for determination of the formation and growth of micro-cracks in carbon-reinforced composite materials under load. There has still further arisen a need to provide an apparatus that enables the determination of the initial formation of macro-cracks in carbon-reinforced composite materials.